1. Field of the invention
The present invention generally relates to a method for testing semiconductor integrated circuit devices, and to a voltage drop power supply circuit suitable for the test. More particularly, the present invention is concerned with a method for testing semiconductor integrated circuit devices in order to detect an initial fault in the devices by applying an acceleration voltage (burn-in voltage) higher than a normal operation voltage to internal structural elements of the devices, and is concerned with a voltage drop power supply circuit provided in the devices and suitable for such a voltage acceleration test. Further, the present invention is concerned with semiconductor integrated circuit devices having voltage drop power supply circuits as described above.
2. Description of the Prior Art
A recent demand to increase the integration density of semiconductor integrated circuit devices requires diminished size of MOS (Metal Oxide Semiconductor) transistors formed on chips of the devices. Such fine MOS transistors have a large electric field between the source and drain, and this results in hot carriers. Such a hot carrier affects the MOS transistors and may damage the MOS transistors, so that the reliability of the MOS transistors is degraded. With the above in mind, an improvement has been proposed in which an external power supply voltage is dropped by means of a voltage drop power supply circuit mounted on a semiconductor integrated circuit devices and by which a dropped voltage is applied to internal circuits of the devices. A decrease in the power supply voltage to be applied to the internal circuits increases the resistance of the MOS transistors to the hot carriers.
A conventional voltage drop power supply circuit can be classified into:
(1) a flat voltage characteristic type circuit in which the output voltage (i.e. the voltage after the voltage drop) does not change when the external power supply voltage changes, or
(2) an external voltage dependent type circuit in which the dropped voltage depends on the external power supply voltage.
The voltage drop power supply circuit of the flat voltage characteristic type (1) is capable of stably generating a constant voltage independent of variations in the external power supply voltage. Hence, the voltage drop power supply circuit of the flat voltage characteristic type is frequently employed, as compared to the voltage drop power supply circuit of the above type (2).
A voltage acceleration test (burn-in test) is known as one of methods for testing semiconductor integrated circuit devices. In the voltage acceleration test, a high voltage (acceleration or burn-in voltage) outside a normal operation voltage range for internal circuits of the semiconductor integrated circuit devices is applied to the internal circuits for a predetermined period. The application of the high voltage does not affect normal transistors of the semiconductor integrated circuit devices. However, deterioration of defective transistors is accelerated by the application of the high voltage. Semiconductor integrated circuit devices having defective transistors which have deteriorated by the application of the high voltage for the predetermined period are discarded. However, even when the external power supply voltage is increased in order to perform the voltage acceleration test, the output voltage after the voltage drop does not reach the operation voltage necessary for the voltage acceleration test in the semiconductor integrated circuit devices having the voltage drop power supply circuits of the flat voltage characteristic type, and hence the voltage acceleration test cannot be substantially performed. Hence, it is required that the voltage acceleration test can be carried out for the semiconductor integrated circuit devices having the voltage drop power supply circuits of the flat voltage characteristic type.
FIG. 1 is a block diagram of a conventional voltage drop power supply circuit provided in a semiconductor integrated circuit device. Referring to FIG. 1, a constant internal voltage supply unit 1, a burn-in voltage (acceleration voltage) supply unit 2, and a regulator unit 3 are formed in a semiconductor integrated circuit device. An external power supply voltage Vcc is applied to the units 1, 2 and 3. The regulator unit 3 outputs an operation voltage (after the voltage drop) to internal circuits of the semiconductor integrated circuit device.
When the external power supply voltage Vcc is within a normal operation voltage range, the constant internal voltage supply unit 1 generates a predetermined constant internal power supply voltage V1, and the burn-in voltage supply unit 2 generates a burn-in voltage V2 lower than the constant internal power supply voltage V1. Hence, the constant voltage V1 is applied to the regulator unit 3, which generates a constant operation voltage V.
When the external power supply voltage Vcc exceeds the normal operation voltage range, the constant voltage V1 is continuously output by the constant internal voltage supply unit 1, while the burn-in voltage V2 proportional to the external power supply voltage Vcc is generated by the burn-in voltage supply unit 2. Since V2&gt;V1, the burn-in voltage V2 is applied to the regulator unit 3. The regulator unit 2 outputs the voltage V dependent on the input voltage V2. In the above manner, when the external power supply voltage Vcc increases and becomes out of the normal operation voltage range, the burn-in voltage V2 proportional to the external power supply voltage Vcc is generated by the burn-in voltage supply unit 2, and the internal power supply voltage, which is generated by the regulator unit 3 and is higher than the normal operation voltage necessary for the transistors, is applied to the internal circuits. Hence, the voltage acceleration test (burn-in test) can be performed.
It is known that dropped voltage (internal power supply voltage) vs. external power supply voltage characteristics shown in FIGS. 2 through 4 depend on the structure of the burn-in voltage supply unit 2. In the voltage characteristics shown in FIGS. 2 through 4, the internal power supply voltage increases in proportion to the external power supply voltage Vcc until the external power supply voltage Vcc reaches a voltage Vcc1. The internal power supply voltage is then maintained at a constant value within an operation voltage range between Vcc1 and Vcc2.
As shown in FIG. 2, the internal power supply voltage changes along a straight line I connecting the origin O and the internal power supply voltage corresponding to the voltage Vcc2 when the burn-in test, in which the external power supply voltage Vcc is increased to a level higher than the voltage Vcc2, is carried out for a semiconductor integrated circuit device having the burn-in voltage supply unit 2 having the voltage characteristics shown in FIG. 2. As shown in FIG. 3, the internal power supply voltage changes along a straight line II parallel to a straight line connecting the origin O and the internal power supply voltage corresponding to the voltage Vcc1 when the burn-in test is carried out for a semiconductor integrated circuit device having the burn-in voltage supply unit 2 having the voltage characteristics shown in FIG. 2. The internal power supply voltage changes along a straight line III (FIG. 4) connecting the origin O and the internal power supply voltage corresponding to the voltage Vcc2. In this case, the internal power supply voltage is equal to the external power supply voltage.
Normally, semiconductor integrated circuit devices include transistors to which the external power supply voltage is applied, and transistors to which an internal power supply voltage generated by a voltage drop power supply circuit is applied. The conventional voltage acceleration test for such semiconductor integrated circuit devices is carried out so that the voltage application condition set during the test does not match the voltage application condition set during the normal operation. For example, the ratio of the internal power supply voltage to the external power supply voltage used during the normal operation is not equal to that used during the voltage acceleration test. In this case, one of the two types of transistors deteriorates faster than the other type of transistors, and a calculation procedure for obtaining the ratios becomes complex and troublesome. As a result, it is very difficult to complete the voltage acceleration test in a short time.
More particularly, the conventional voltage acceleration test cannot be performed at a ratio of the internal power supply voltage to the external power supply voltage (Vcc1-Vcc2) not equal to that set during the normal operation (in the case shown in FIG. 3). Even when the same ratio can be set, the voltage acceleration test can be performed with respect to only a limited part of the normal operation voltage range. In the voltage characteristic shown in FIG. 2, the voltage acceleration test can be performed at the same ratio as the normal operation in which the external power supply voltage Vcc is equal to Vcc2. It will be noted that the voltage Vcc2 is the upper limit of the normal operation voltage range. In the voltage characteristic shown in FIG. 4, the voltage acceleration test can be performed at the same ratio as the normal operation in which the external power supply voltage Vcc is equal to Vcc1. It will be noted that the voltage Vcc1 is the lower limit of the normal operation voltage range. Hence, the reliability of the acceleration test is not good.